In general, conversion of light paraffins and light olefins to more valuable cuts is very lucrative to the refining industries. This has been accomplished by alkylation of paraffins with olefins, and by polymerization of olefins. One of the most widely used processes in this field is the alkylation of iso-butane with C3-C5 olefins to make gasoline cuts with high octane number using sulphuric and hydrofluoric acids. This process has been used by refining industries since the 1940's. The process was driven by the increasing demand for high quality and clean burning high octane gasoline.
Commercial paraffin alkylation processes in modern refineries use either sulphuric acid or hydrofluoric acid as catalyst. Both of these processes require extremely large amounts of acid to fill the reactor initially. The sulphuric acid plant also requires a significant daily withdrawal of spent acid for off-site regeneration. Then the spent sulphuric acid is incinerated to recover SO2/SO3 and fresh acid is prepared. The necessity of having to handle a large volume of used acid is considered a disadvantage of the sulphuric acid based processes. On the other hand, an HF alkylation plant has on-site regeneration capability and daily make-up of HF is orders of magnitude less. However, the aerosol formation tendency of HF presents a potentially significant environmental risk and some regard a HF alkylation process to be a less safe process than a H2SO4 alkylation process. Modern HF processes often require additional safety measures such as water spray and catalyst additive for aerosol reduction to minimize the potential hazards.
Although these catalysts have been successfully used to economically produce the best quality alkylate, the need for safer and environmentally-more friendly catalyst systems has become an issue to the industries involved. The ionic liquid catalyst of the present invention fulfills that need.
The progressive trend towards lower sulphur automotive fuels has resulted in an increased demand for hydrogen in crude oil refining for desulphurization. Smaller refineries typically have a single source of hydrogen—the reformer. Although hydrogen made with conventional Platinum/Rhenium reforming catalysts can be increased by lowering operating pressure, there is an attendant increase in catalyst fouling, which shortens catalyst run length. There are practical and economic limits to how far pressure can be lowered on semi regenerative reformers before the costs and disruptions of frequent shutdowns for catalyst regeneration become prohibitive. Typically, refiners limit run lengths to no less than 6 months, which in effect limits operating pressure to above 250 psig.
Ionic liquids are liquids that are composed entirely of ions. The so-called “low temperature” Ionic liquids are generally organic salts with melting points under 100 degrees C., often even lower than room temperature. Ionic liquids may be suitable for example for use as a catalyst and as a solvent in alkylation and polymerization reactions as well as in dimerization, oligomerization acetylation, metatheses, and copolymerization reactions.
One class of ionic liquids is fused salt compositions, which are molten at low temperature and are useful as catalysts, solvents and electrolytes. Such compositions are mixtures of components which are liquid at temperatures below the individual melting points of the components.
Ionic liquids can be defined as liquids whose make-up is entirely comprised of ions as a combination of cations and anions. The most common ionic liquids are those prepared from organic-based cations and inorganic or organic anions. The most common organic cations are ammonium cations, but phosphonium and sulphonium cations are also frequently used. Ionic liquids of pyridinium and imidazolium are perhaps the most commonly used cations. Anions include, but not limited to, BF4—, PF6—, haloaluminates such as Al2Cl7— and Al2Br7—, [(CF3SO2)2N)]—, alkyl sulphates (RSO3—), carboxylates (RCO2—) and many other. The most catalytically interesting ionic liquids for acid catalysis are those derived from ammonium halides and Lewis acids (such as AlCl3, TiCl4, SnCl4, FeCl3 . . . etc). Chloroaluminate ionic liquids are perhaps the most commonly used ionic liquid catalyst systems for acid-catalyzed reactions.
Examples of such low temperature ionic liquids or molten fused salts are the chloroaluminate salts. Alkyl imidazolium or pyridinium chlorides, for example, can be mixed with aluminum trichloride (AlCl3) to form the fused chloroaluminate salts. The use of the fused salts of 1-alkylpyridinium chloride and aluminum trichloride as electrolytes is discussed in U.S. Pat. No. 4,122,245. Other patents which discuss the use of fused salts from aluminum trichloride and alkylimidazolium halides as electrolytes are U.S. Pat. Nos. 4,463,071 and 4,463,072.
U.S. Pat. No. 5,104,840 describes ionic liquids which comprise at least one alkylaluminum dihalide and at least one quaternary ammonium halide and/or at least one quaternary ammonium phosphonium halide; and their uses as solvents in catalytic reactions.
U.S. Pat. No. 6,096,680 describes liquid clathrate compositions useful as reusable aluminum catalysts in Friedel-Crafts reactions. In one embodiment, the liquid clathrate composition is formed from constituents comprising (i) at least one aluminum trihalide, (ii) at least one salt selected from alkali metal halide, alkaline earth metal halide, alkali metal pseudohalide, quaternary ammonium salt, quaternary phosphonium salt, or ternary sulfonium salt, or a mixture of any two or more of the foregoing, and (iii) at least one aromatic hydrocarbon compound.
Other examples of ionic liquids and their methods of preparation may also be found in U.S. Pat. Nos. 5,731,101; 6,797,853 and in U.S. Patent Application Publications 2004/0077914 and 2004/0133056.
In the last decade or so, the emergence of chloroaluminate ionic liquids sparked some interest in AlCl3-catalyzed alkylation in ionic liquids as a possible alternative. For example, the alkylation of isobutane with butenes and ethylene in ionic liquids has been described in U.S. Pat. Nos. 5,750,455; 6,028,024; and 6,235,959 and open literature (Journal of Molecular Catalysis, 92 (1994), 155-165; “Ionic Liquids in Synthesis”, P. Wasserscheid and T. Welton (eds.), Wiley-VCH Verlag, 2003, pp 275).
Aluminum chloride-catalyzed alkylation and polymerization reactions in ionic liquids may prove to be commercially viable processes for the refining industry for making a wide range of products. These products range from alkylate gasoline produced from alkylation of isobutane and isopentane with light olefins, to diesel fuel and lubricating oil produced by alkylation and polymerization reactions.
The alkylation of iso-butane with butene is a well established process in the oil and gas industry. Typical sulphur levels in gasoline are 5-30 ppm depending on the operating conditions. Alkylate sulphur comes from FCC butene, the amount of sulphur varies by refinery, but are typically 20-100 ppm. In order to lower the alkylate sulphur level, mercaptan sulphur can be removed before alkylation by caustic wash, but this process does not remove disulphides. Sulphur can be removed from alkylate during a finishing step by using ionic liquid catalysts. The ionic liquid based on aluminium chloride and cuprous chloride can remove sulphur without degrading alkylate.